Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective glass cladding. Presently there are several known processes for fabricating preforms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027, issued in the names of J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Telephone Laboratories, Inc., has been found to be one of the most useful because the process enables large scale production of preforms which yield very low loss optical fiber.
During the fabrication of preforms by the MCVD process, reactant-containing gases, such as SiCl.sub.4 and GeCl.sub.4 are passed through a rotating substrate tube suspended between the headstock and tailstock of a lathe. A torch assembly, which heats the tube from the outside as the gases are passed therethrough, traverses the length of the tube in a number of passes, and provides the heat for the chemical reactions and deposition upon the inner wall of the tube. The torch assembly also supplies the heat for collapsing the tube to form a rod, and, in subsequent operations, for collapsing an overclad tube onto the rod, as explained in the aforementioned Mueller et al.--943 application. In the current manufacture of preforms, the torch is mounted on a carriage which is a solid structure supported and guided on the lathe or machine bed. The guidance of the carriage along a specific path is usually accomplished through the use of a typical three sided gib and way system, with the carriage having rolling or sliding elements attached and in contact with the tops, sides, and bottoms of a dual way system. Linear guide rails having various cross-sections for rolling and sliding elements and mounted to the bed may be used as an alternative. In the systems as currently used, the sliding or rolling elements on the carriage are in direct contact with the bed of the lathe or machine or with the ways. In all such systems, the movement of the carriage and the physical contact between it and the bed requires lubrication to eliminate wear and friction. An initial "stick-skip" condition must be overcome during the start of carriage motion which is a result of the friction, and the friction can also induce "jerk" in the movement of the carriage along the bed. In addition, the friction can cause or induce, over a period of time, freeplay in the system as a result of wear. Thus, where a smooth uniform velocity of the torch down the length of the tube is a necessity for uniformity of heating and deposition and, ultimately, a uniformity of product, the friction effects can, and most often do, cause a non-uniform velocity profile, and, as a consequence, non-uniformity of heating and deposition, which result in non-uniformity of product. In present day practice, friction is overcome, at least in part, through the use of lubricants which, during a production run, become a contaminant to the process and spread throughout the machine. This, in turn, necessitates frequent cleaning of the apparatus which is detrimental to the goal of substantially continuous production. Further, the lubricant does not completely eliminate the stick-slip and jerk problems which, as pointed out in the foregoing, most often lead to a nonuniform velocity profile.
The related U.S. patent application Ser. No. 09/500,154 is directed to a carriage guidance system that substantially eliminates physical contact between the carriage and lathe bed and, hence, overcomes most if not all of the aforementioned problems. The arrangement shown in that application is a hydrostatic guidance and support system for the movable carriage upon which the torch for the MCVD process is mounted. The carriage, as used on the MCVD lathe, is equipped with integral air bearing components which, in their geometry, match the lathe bed cross-section. Fluid, such as air, under pressure, is delivered to the bearings which, under pressure of the air or whatever fluid is used, in use, cause the carriage to float in spaced relationship to the lathe, thereby producing a nearly friction free support and guide for the carriage, which results in a smooth velocity profile, which, in turn, produces a drastic improvement in the quality (and quantity) of the MCVD product. The terms "fluid" and "air" will be used interchangeably hereinafter.
In greater detail, the carriage comprises a top plate to which the torch is mounted, first and second side walls depending from the top plate, and first and second inward facing guidance members in the form of flanges extending inwardly from the bottoms of the side walls. The top plate has four downwardly oriented threaded bores extending therethrough which are spaced to overlie the rails or ways of the lathe bed. Threaded studs are mounted in the bores, each stud having a partially spherical end face which fits into a hole having a spherically shaped bottom in a porous pad member thereby creating a ball joint to hold the member in place, especially while in motion. In like manner, each of the side walls has similar bores aligned with the sides of the lathe rails and in which similar studs are mounted which hold similar porous pads. Each of the flanges has a pair of bores therein for studs which also hold porous pads, beneath the ways or rails of the lathe.
On each of the side walls is mounted an air manifold having at least one air input, and six outputs having needle valves mounted therein. Thus, when pressurized air is supplied from a source to the manifold, each needle valve has a quantity of pressurized air emerging therefrom. The output of each needle valve is supplied by means of suitable tubing, to a porous pad, and each manifold supplies air to six of the pads of which there are twelve in all. Each pad, which preferably comprises porous graphite and which has a smooth porous face, has an input to which the pressurized air from the manifold is supplied. With all of the pads in place and with its pressurized air from the source being at an adjusted value of, for example, fifty-five (55) pounds per square inch, the needle valves and the threaded studs are used to fine tune the air pressure to the point where the carriage floats free of contact with the lathe bed, but properly centered on all axes. The carriage, which may be moved longitudinally by any of a number of drives, such as a worm drive, a rack and pinion drive, or a belt drive, for example, is then movable substantially without friction along the lathe bed, thereby insuring a substantially uniform velocity profile.
Inasmuch as there is no contact between the carriage and the lathe bed, lubrication and contamination of the MCVD process are eliminated.
The hydrostatic carriage arrangement of the application eliminates most of the maintenance associated with existing mechanical linear slide systems, the clogging of the lubricants in the elements, the contaminants to the process area, and velocity uniformities.
Also, because friction is substantially eliminated, the prime mover of the carriage, e.g., rack and pinion, having less of a load thereon, may be downsized in terms of the power requirements necessary to move the carriage.
Heretofore, in the prior art carriage arrangements wherein rolling or sliding elements on the carriage are in physical contact with the rails, for example, of the lathe bed, the movements of the carriage over time create wear on the moving surfaces. The wear is generally non-uniform and may progress to the point where gapping between the moving elements occurs. As the carriage traverses along the length of the bed, areas of binding or loosening may be encountered due to the wear. If a worn condition is present, the maintenance is usually directed to eliminating binding at the tightest point, which means that there will be portions of the carriage traverse that are loose. Some prior art arrangements make use of pre-loaded pivots or other spring loaded systems to maintain a uniform contact force between the moving elements. However, the number of components, which may include moving components, and their complexity impact the effectiveness of the system, and the velocity profile offer time of the carriage is directly depending upon the aforementioned factors.
The floating carriage arrangement of the aforementioned Mueller application overcomes, as pointed out, most of the problems of sticking and binding, provided the lathe bed has not been previously distorted through excess wear. Ideally, it would be a near perfect solution if the existing lathes were replaced with ones having no wear, rail bowing, or the like, but such a replacement would not be economically feasible. It would be preferable if the floating carriage arrangement could be modified to match existing rails and the like of existing lathe beds, thus making retrofit possible.